1. Introduction
Myocardial infarction (MI), stroke, transient ischemic attacks (TIA) and pulmonary embolism (PE) are major causes of morbidity and mortality worldwide. These life-threatening clinical events are mostly caused by thrombi, which can be located in different vessels spread all over the body and can be of different size and composition. The origin of stroke or TIA can for example be a thrombus in the left atrium (LA) of the heart or in one of the big arteries between heart and brain like the carotid artery. In case of PE a venous thrombosis, often situated in the lower legs, can be the cause.
In a growing thrombus the final common step of platelet aggregation is characterized by the binding of activated glycoprotein IIb/IIIa to blood fibrinogen resulting in a crosslink inside the platelets. Design and development of glycoprotein IIb/IIIa inhibitors (Scarborough R. M., Gretler D. D., J. Med. Chem. 2000, 43, 3453-3473) has been of considerable interest in pharmacological research with respect to anti-platelet and anti-thrombotic activity.
However, health care professionals are in need not only for compounds that prevent thrombosis in an acute care setting, but also for a satisfactory method of imaging thrombi down to a size of 1 mm3. More particularly, a thrombus imaging is important because clinical applications such as thrombolytic intervention require that the thrombus formation site is identified and enables the monitoring of therapy effects. In this way a thrombus imaging can help to avoid unnecessary prophylactic applications and therewith anticoagulant treatments which are associated with risks (e.g. severe bleedings due to the reduced coagulation capacity).
The patient population which may benefit from such a diagnostic procedure is huge. According to the “Heart disease and Stroke Statistics—2010 Update” of the American Heart Association 17.6 million people suffered from coronary heart disease only in the USA. Every year an estimated 785,000 Americans will have a new coronary attack, and approximately 470,000 will have a recurrent attack. Every year about 795,000 patients experience a new or a recurrent stroke. About 610,000 of these are first attacks. Of all strokes, 87% are ischemic, most of them due to a thromboembolic cause (Lloyd-Jones, D. et al., Circulation, 2010, 121(7): p. e46-215). The incidence of transient ischemic attack (TIA) in the United States has been estimated to be approximately 200,000 to 500,000 per year, with a population prevalence of 2.3%, which translates into about 5 million people (Easton, J. D. et al., Stroke, 2009, 40(6): p. 2276-2293). Individuals who have a TIA have a 90-day risk of stroke of 3.0% to 17.3% and a 10-year stroke risk of 18.8%. The combined 10-year stroke, myocardial infarction, or vascular death risk is even 42.8% (Clark, T. G., M. F. G. Murphy, and P. M. Rothwell, Journal of Neurology, Neurosurgery & Psychiatry, 2003. 74(5): p. 577-580).
Imaging is forefront in identifying thrombus. Currently, thrombus imaging relies on different modalities depending on the vascular territory. Carotid ultrasound is used to search for carotid thrombus, transesophageal echocardiography (TEE) searches for cardiac chamber clot, ultrasound searches for deep vein thrombosis, and CT has become the gold standard for PE detection.
2. Description of the Prior Art, Problem to be Solved and its Solution
Despite the success of these techniques, there continues to be a strong need for a molecular imaging solution for thrombus detection and monitoring: first, there are certain vascular territories that are underserved. For instance, despite best imaging efforts some 30% to 40% of ischemic strokes are “cryptogenic,” that is, of indefinite cause, or in other words, the source of the thromboembolism is never identified (Guercini, F. et al., Journal of Thrombosis and Haemostasis, 2008. 6(4): p. 549-554). Underlying sources of cryptogenic stroke include atherosclerosis in the aortic arch or intracranial arteries. Plaque rupture in the arch or other major vessels, in particular, is thought to be a major source of cryptogenic strokes and can be difficult to detect with routine methods. Recent clinical trial data from transesophageal Echocardiography (TEE) studies showed that the presence of thickened vessel wall in the aortic arch was not predictive of ischemic stroke, although ulcerated aortic arch plaques are associated with cryptogenic stroke. A thrombus-targeted molecular imaging approach could potentially identify clot in the presence of atherosclerotic plaque. Finally, there is a need for an approach wherein a single modality could be used to identify thrombus throughout the body. For instance, in a TIA or stroke follow-up, currently multiple examinations are required to search for the source of the embolus (Ciesienski, K. L. and P. Caravan, Curr Cardiovasc Imaging Rep., 2010. 4(1): p. 77-84).
As already mentioned the therapeutic application of glycoprotein IIb/IIIa inhibitors (Scarborough R. M., Gretler D. D., J. Med. Chem. 2000, 43, 3453-3473) has been of considerable interest in the past. Meanwhile three glycoprotein IIb/IIIa antagonists are commercially available: a recombinant antibody (Abciximab), a cyclic heptapeptide (Eptifibatid) and a synthetic, non-peptide inhibitor (Tirofiban). Tirofiban (brand name AGGRASTAT) belongs to the class of sulfonamides and is the only synthetic, small molecule among the above mentioned pharmaceuticals. Duggan et. al., 1994, U.S. Pat. No. 5,292,756 disclose sulfonamide fibrinogen receptor antagonist as therapeutic agents for the prevention and treatment of diseases caused by thrombus formation.
One attempt to fulfill the need of thrombus imaging is represented by the SPECT tracer apticide (AcuTect®). Apticide is a Tc-99m labeled peptide specifically binds to the GPIIb/IIIa receptor and is used for imaging of thrombi in a mammalian body. Dean et al., 1996, U.S. Pat. No. 5,508,020, disclosed radiolabeled peptides, methods and kits for making such peptides to image sites in a mammalian body labeled with technetium-99m via Tc-99m binding moieties. Dean and Lister-James describe peptides that specifically bind to GPIIb/IIIa receptors on the surface of activated platelets (U.S. Pat. No. 5,645,815; U.S. Pat. No. 5,830,856 and U.S. Pat. No. 6,028,056). Apticide was approved for the detection of deep vein thrombosis. The resolution reached by the technetium labeled peptide was found not to be satisfactory due to unspecific binding and high background.
Novel highly specific non-peptide glycoprotein IIb/IIIa antagonists have been described in the prior art (Damiano et. al., Thrombosis Research 2001 104, 113-126; Hoekstra, W. J., et al., J. Med. Chem., 1999, 42, 5254-5265). These compounds have been known to be GPIIb/IIIa antagonist, effective as therapeutic agents with anti-platelet and anti-thrombotic activity (see WO9508536, WO9629309, WO9733869, WO970160813, U.S. Pat. No. 6,515,130). The potential use of glycoprotein IIb/IIIa antagonists as contrast agents is also proposed (see US 2007/0189970 A1). However, a thrombus imaging with these compounds has not been demonstrated.
The F-18 radiolabeled compounds and their precursors described in this invention surprisingly show high metabolic stability, low protein binding and fast elimination. A non-invasive PET imaging of very small thrombi in vivo with an 18F-labeled tracer has been successfully demonstrated.
The images show strong signals in arterial and venous thrombi. In contrast to Apticide (U.S. Pat. No. 5,645,815; U.S. Pat. No. 5,830,856 and U.S. Pat. No. 6,028,056 there is no background visible in the whole body except the excreting organs liver and kidney. Most surprisingly, even very small thrombi (thickness <1 mm) can be detected and show a bright signal in the image. Detection of such small thrombi has not been described so far. The thrombi shown with Apticide had a size of 89 plusminus 26 mg [mean plusminus SEM] (Lister-James, J., et al., J Nucl Med, 1996. 37(5): p. 775-81).
The imaging of small thrombi is particularly important with regard to thromboembolic diseases such as myocardial infarction, pulmonary embolism, stroke and transient ischemic attacks. Furthermore, a sensitive thrombus PET marker can be used for the regular health monitoring of cardiovascular risk patients, or the diagnosis of life threatening diseases such as aortic aneurism, chronic thromboembolic pulmonary hypertension (CETPH), atrial fibrillation and coronary thrombosis.
The in vivo clot-to-blood ratio of the disclosed compounds lies in a high, especially desirable range, which is a property that has never been observed before. In fact, it is the decisive property of the new compounds that is responsible for the clear advantage over the current state of the art compounds and enables a much lower radioactive dose to be administered to a potential patient (only 15 MBq needed for monkeys, which is at least 20-fold less the dose used in the closest prior art US 2007/0189970 A1).